Particle characterization apparatus and method

ABSTRACT

An apparatus is provided for determining particle characteristics, in which a flow path is generated containing particles to be analyzed. A light detection system detecting light received from a measurement zone which has been scattered by the particles. A time duration for which a particle remains in the measurement zone is measured to determine an effective aerodynamic particle diameter and a peak detected received light intensity is measured to determine an effective optical particle diameter. A further particle parameter is also obtained relating to the shape and/or density of the particle. This approach enables more information than only a particle size to be obtained using a single-stage optical analysis system. The additional information may be used to characterize the particles more accurately.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for characterizingparticles.

BACKGROUND OF THE INVENTION

It is well known that it is desirable to monitor pollution levels, suchas particulate pollutants. Various sensing devices are known, which forexample provide a particle concentration level for particles below acertain size. Optical particle sensing approaches are for example knownbased on optical scattering.

It is also of interest to know the particle size or particle sizedistribution of a pollutant for example to identify the pollutant andhence the cause or source of the pollution.

Various particle sizing techniques are known. These techniques aim tomeasure a particle equivalent diameter, and there are differentdefinitions for this equivalent diameter. For example, a particleeffective optical diameter (d_(op)) is an equivalent diameter reportedfrom a Mie light scattering measurement technique. A particle effectiveaerodynamic diameter (d_(ae)) is the equivalent diameter of a sphericalparticle with standard density which settles at the same terminalvelocity as the particle of interest.

There is a difference between the two diameters, which results from theshape (e.g. spherical vs. non-spherical) and density (as compared to astandard density of 1 g/cm³) of the particle.

It would be of interest to obtain additional information about theparticle characteristics other than a simple equivalent diameter. Forexample, efforts have been made to measure a sub-micron particle shapefactor and density in scientific application scenarios. Typically, twoor more equivalent particle diameters are measured by correspondingmeasurement techniques connected in series along a gas flow. Thesecumbersome instruments usually require a large space to set up and theyrequire skilled operators.

The article “Laboratory and Ambient Particle Density Determinationsusing Light Scattering in Conjunction with Aerosol Mass Spectrometry” ofEben S. Cross et. al. in Aerosol Science and Technology, vol. 41. No. 4,5 Mar. 20007 pages 343-359 discloses a mass spectrometer with an opticalstage for determining a vacuum aerodynamic diameter from a time offlight measurement and a particle optical diameter from a scatteredlight intensity measurement.

US 2009/09249 discloses a system for estimating size segregated aerosolmass concentration. It combines scattered light intensity measurementand time of flight measurement as a particle traverses an interrogationbeam.

There remains a need for an analysis system which enables moreinformation about particles to be obtained than a simple diametermeasurement, but with a low cost and compact arrangement.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to examples in accordance with an aspect of the invention,there is provided an apparatus for determining particle characteristics,comprising:

a flow system for generating a flow path between an inlet and an outletalong which an accelerating flow is to be provided containing particlesto be analyzed;

a light source for providing light to the flow path, wherein theenvelope of the light defines a measurement zone, the measurement zonehaving a length of more than 1 cm;

a light detector for detecting light received from the measurement zonewhich has been scattered by the particles; and

a controller for analyzing the detected received light,

wherein the controller is adapted to:

-   -   determine a time duration for which a particle remains in the        measurement zone, and thereby determine an effective aerodynamic        particle diameter;    -   determine a peak detected received light intensity and thereby        determine an effective optical particle diameter;    -   determine a further particle parameter relating to the shape        and/or density of the particle.

This apparatus enables multiple parameters to be obtained about detectedparticles using a single optical analysis system. In particular, botheffective optical and aerodynamic diameters are obtained, as well asfurther shape or density information. This is possible by making use ofa relatively long measurement zone so that particle transit timeinformation is obtained as an additional variable to the lightintensity. The use of an accelerating flow contributes to the differencein transit time (i.e. the time the particles are resident in themeasurement zone) for particles with different aerodynamic diameters,hence enables particle sizing.

The measurement zone length may be between 1 cm and 6 cm.

The additional information may be used to determine particle type andcomposition.

The light source is preferably a laser.

In a basic implementation, the controller may be adapted to determinethe further particle parameter as a ratio between a particle shapeparameter and a particle density. This ratio may be obtained directlyfrom the two effective particle diameters without any furthermeasurements.

In another implementation, the controller is adapted to:

analyze a level of variation of the detected received intensity overtime and thereby derive a particle shape parameter as a first furtherparticle parameter; and

determine a particle density as a second further particle parameter.

The variation of the peak intensity may be caused by rotation of anon-spherical particle during the scattering measurement. By measuringthe level of signal variation, a measure may be obtained relating to thelevel of uniformity of the particle shape.

The apparatus may further comprise:

a first polarizer between the light source and the measurement zone; and

a second polarizer between the measurement zone and the light detector,wherein the second polarizer has a first portion with a matchingpolarization to the first polarizer and a second portion with anorthogonal polarization to the first polarizer.

In this way, it becomes possible to determine the proportion of lightwhich has undergone a polarization shift compared to light that has notundergone a polarization shift, and hence to differentiate betweenscattering amplitudes in the Mie scattering S-matrix, which in turndepends on the particle shape.

For example, the controller may be adapted to:

analyze the peak detected received light intensity through the first andsecond portions of the second polarizer to derive a particle shapeparameter as a first further particle parameter; and

determine a particle density as a second further particle parameter.

The apparatus may further comprise:

an outer enclosure;

a fan connected to the outlet;

a filter arrangement coupled to a further pair of inlets; and

a flow deflector arrangement for controlling the flow from the filterarrangement and for controlling the flow path.

The filter arrangement is used to control the flow. For example, bysuitable design of the filter arrangement the flow along the flow pathmay be controlled to introduce a single particle at a time into themeasurement zone. This for example involves diluting the particleconcentration introduced from the inlet by streams of clean air from thefilter arrangement.

The filter arrangement may comprise first and second filters on oppositesides of the flow path, and the flow deflector arrangement comprisescorresponding first and second flow deflectors.

The apparatus may be adapted to provide a uniform acceleration of theflow along the flow path within the measurement zone.

Examples in accordance with another aspect of the invention provide amethod for obtaining characteristics, comprising:

generating an accelerating flow containing particles between an inletand an outlet;

controlling a light source to provide light to the flow path, whereinthe envelope of the light defines a measurement zone, the measurementzone having a length of more than between 1 cm;

detecting light received from the measurement zone which has beenscattered by the particles;

determining a time duration for which a particle remains in themeasurement zone, and thereby determining an effective aerodynamicparticle diameter;

determining a peak detected received light intensity and therebydetermining an effective optical particle diameter; and

determining a further particle parameter relating to the shape and/ordensity of the particle.

This method makes use of a prolonged intensity measurement so that bothtime duration and intensity become significant, and this provides twomeasurements from a single optical analysis stage. An accelerating flowfield ensures that particles of different aerodynamic sizes havedifferent transit times in the measurement zone. This in turn enablesmore information than just a particle diameter to be derived.

The method may comprise: determining the further particle parameter as aratio between a particle shape parameter and a particle density.

The method may instead comprise:

analyzing a level of variation of the detected received intensity overtime and thereby deriving a particle shape parameter as a first furtherparticle parameter; and

determining a particle density as a second further particle parameter.

The method may instead comprise:

providing polarization of the light before the measurement zone using afirst polarizer;

providing polarization of the light after the measurement zone using asecond polarizer having a first portion with a matching polarization tothe first polarizer and a second portion with an orthogonal polarizationto the first polarizer;

analyzing the peak detected received light intensity through the firstand second portions of the second polarizer to derive a particle shapeparameter as a first further particle parameter; and

determining a particle density as a second further particle parameter.

The method may be used to differentiate between different types ofpollen, e.g. in an allergy detection device. The particles described inthis disclosure may be pollen. The different particle characteristicsmay be used to differentiate between different types of pollen.

A filtered air flow may be provided towards the measurement zone therebyto control the flow path. This may be performed to focus the flow path,to generate the desired accelerating flow field and to dilute theparticle number concentration in the flow from the inlet to ensure thata single particle passes the measurement zone at a time. A uniformacceleration of the flow along the flow path may for example be providedwithin the measurement zone.

The invention may be implemented at least in part in software.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1 shows a first possible way to obtain information relating toparticles in addition to the particle diameter;

FIG. 2 shows a second possible way to obtain information relating toparticles in addition to the particle diameter;

FIG. 3 shows a first example of a particle size determining apparatusand shows the parts relating to the optical system;

FIG. 4 shows an example of the collected light intensity signal;

FIG. 5 shows the parts of the apparatus relating to flow control;

FIG. 6 shows the flow paths within the apparatus;

FIG. 7 shows the velocity profile within the measurement zone;

FIG. 8 shows the effect of particle density and hence the particleaerodynamic diameter on the velocity reached in the accelerating flowfield;

FIG. 9 shows the terminal velocity versus the particle diameter.

FIG. 10 shows the sensitivity of the terminal velocity versus theparticle diameter.

FIG. 11 shows the pulse width versus the particle diameter.

FIG. 12 shows the sensitivity of the pulse width versus the particlediameter;

FIG. 13 shows a collected signal for a non-spherical particle;

FIG. 14 shows a collected signal for a spherical particle;

FIG. 15 shows an apparatus which uses polarization to enabledetermination of a shape parameter;

FIG. 16 shows an example of the collected light intensity for anon-spherical particle using the apparatus of FIG. 15;

FIG. 17 shows parameters associated with the optical arrangement of FIG.3 and shows how decreasing the angle of incidence θ_(mc) can increaseirradiance while maintaining the length of the measurement zone L; and

FIG. 18 shows a method of determining particle characteristics.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides an apparatus for determining particlecharacteristics, in which a flow path is generated containing particlesto be analyzed, in which an accelerating flow is provided. A lightdetection system detects light received from a measurement zone whichhas been scattered by the particles. A time duration for which aparticle remains in the measurement zone is measured to determine aneffective aerodynamic particle diameter and a peak detected receivedlight intensity is measured to determine an effective optical particlediameter. A further particle parameter is also obtained relating to theshape and/or density of the particle.

This approach enables more information than only a particle size to beobtained using a single-stage optical analysis system. The additionalinformation may be used to characterize the particles more accurately.

FIGS. 1 and 2 show examples of possible configurations for obtaininginformation additional to an equivalent diameter measurement.

FIG. 1 shows how multiple measurements may be used in sequence to obtaina particle density measurement.

The system comprises a differential mobility analyzer 10 to which asample flow 12 is provided. This provides an electrical mobilitydiameter EMD as its output. The flow passes to an optical particlecounter 14 which provides an optical diameter OD as its output. The twooutputs are processed by processor 16 which provides a densitymeasurement D.

FIG. 2 shows how multiple measurements may be used in sequence to obtaina shape factor (which is an indication of how far the shape deviatesfrom a perfect sphere).

The system comprises an aerosol particle mass analyzer 20 to which asample flow 22 is provided. This provides an aerosol mass AM as itsoutput. The flow passes to a scanning mobility particle sizer 24 whichprovides an electrical mobility diameter EMD as its output. The twooutputs are processed by processor 26 which provides a shape factormeasurement SF.

A scanning mobility particle sizer essentially performs the samefunction as a differential mobility analyzer but it can also selectparticles with multiple electrical mobility values by changing theelectric field intensity.

These approaches thus require complicated apparatus in order to enhancethe basic measurement of effective diameter.

The invention is instead based on the use of a single optical stage witha wide light source beam such as a laser beam to measure particleeffective optical diameter (based on the intensity of a pulse receivedby the photon detector) and particle transit time and/or terminalvelocity (based on the width of the pulse) in a steady stateaccelerating flow field. By steady state is meant that the flowconditions are temporally constant, i.e. the flow has the constantvelocity (and acceleration) at different points along the flow path overtime.

The wide light source beam defines a measurement zone. In particular,the envelope the light source beam determines the length of themeasurement zone. This means that the transit time through themeasurement zone can be determined based on analysis of reflected light,and does not require any additional timing measurements.

Thus, separate components are not needed for a time of flightmeasurement. Only an incident beam, for example generated by a laserdiode and beam shaping optics, is needed.

A pre-defined relationship may be used to convert the transit timeand/or terminal velocity to an effective aerodynamic diameter. Thetransit time is more easy to measure as it related directly to thesignal width. The effective aerodynamic and optical diameters may thenbe processed to yield additional information about the particle, such asone or more of the particle density, shape factor, type and composition.

By way of example, a laser beam with width 4 cm is wide enough toidentify differences in particle terminal velocity and flight timeinduced by differences in particle effective aerodynamic diameter, andhence particle density and composition. More generally, the measurementzone has a length more than 1 cm, for example a spatial length in therange 1 cm to 6 cm. The larger the length, the greater the sensitivity,so there is a trade-off between sensitivity and sensor size.

This additional information can thus be obtained without requiringmultiple sensing instruments and enables portable sensors to obtainparticle size, type and composition with reasonable accuracy in a quickand cost-effective manner.

The invention combines optical sensing with flow control.

FIG. 3 shows the optical sensing features.

The optical apparatus comprises an inlet 30 which connects to theparticle source. A flow path is defined to an outlet 32 which isconnected to a negative pressure source 33 such as a fan, to control theflow.

A laser diode 34 provides an illumination beam, which is collimated bycollimator lens 36 and then illuminates a length of the flow path. Theenvelope of the laser signal defines the length of the flow path whichin turn defines a measurement zone. A photon detector 38 such as anavalanche photodiode collects scattered light after focusing by lens 40.

The light for example has a top-hat intensity profile. A lenslet arrayor a Powell lens may be used to convert a Gaussian laser output intoincident light with a uniform intensity.

The photon detector signal is provided to a controller 41, which alsocontrols the laser source 34 and the fan 33. The controller outputs theeffective optical diameter d_(op) the effective aerodynamic diameterd_(ac) and one or more further parameters such as a shape factor χ and adensity ρ_(p) or a ratio of such values. It may also output anidentification of the particle type by mapping the particlecharacteristics to known pollutants using a database.

FIG. 4 shows an example of a signal recorded by the photon detector 38over time. The pulse relates to the transit of a single particle. It hasa maximum intensity I_(max) and a time duration Δt between times t₁ andt₂. The measurement zone corresponds to the position of the particlebetween times t₁ and t₂.

The light is collected at a pre-defined angle. The light intensitysignal of FIG. 4 is converted to digital form using an analog to digitalconverter and recorded as a time series, within the controller 41.

Due to the constraints of laser power and analog to digital conversionresolution and signal to noise ratio, a low angle θ between the incidentlaser light and particle beam is used so that the laser power isconcentrated into a small area but a sufficient length of illuminationof the measurement zone is enabled. This is explained further below. Theuse of an avalanche photon detector enables an increase in thesensitivity of the scattered light intensity measurement.

The width of the pulse Δt in FIG. 4, i.e. the transit time, relates tothe velocity of the particle in the laser beam, which is determined bythe aerodynamic diameter of the particle, while the height of the signalI_(max) represents the light intensity that is scattered off theparticle, which is proportional to the effective optical diameter of theparticle.

Thus, the single optical measurement provides multiple sources ofinformation concerning the particle characteristics. In particular, byanalyzing the two effective diameters, one or more of the particledensity, shape factor and particle type can be determined.

FIG. 5 shows the flow control apparatus.

It shows the inlet 30 and outlet 32 of FIG. 3. The inlet is atatmospheric pressure, and the outlet is connected to the fan, such as acentrifugal fan (with a higher static pressure at low flow rate comparedto a coaxial fan). The inlet and outlet pass into an enclosure 42, andfilters 44 are provided within the enclosure, in particular on oppositesides of the flow channel.

The filters 44 are open to the exterior, so that they present a flowpath from the outside to the inside of the enclosure 42. Thus, the flowout of the enclosure 42 is balanced by the flow in through the twofilters and the flow through the inlet 30. The flow rate through theinlet 30, and thus the dilution rate of the incoming particle-laden airflow, depends on the characteristics of the filters, e.g. size,thickness, resistance coefficient etc.

Flow deflectors 46 are provided for shaping the flow streamlines tofocus the particle trajectory from the inlet.

The design of the aerodynamic flow control apparatus part aims togenerate a steady state accelerating flow field inside the measurementzone.

Particles with different aerodynamic diameter accelerate to differentextent in an accelerating flow. This property is thus used to performparticle sizing with respect to aerodynamic diameter. Using a uniformflow field cannot achieve this goal, as all particles will settle to asame velocity profile in the measurement zone, and thus the same transittime.

The filters create a clean environment in the measurement zone toeliminate contamination by unwanted particles during light scatteringmeasurements. This is especially helpful at each start-up of the systemafter it has been idle for a long time. They also dilute the incomingparticle-laden air flow to ensure that, during any time, only oneparticle is present in the measurement zone (i.e. within the laser beampath). The dilution rate can be adjusted by changing the flow resistance(i.e. the material or thickness) of the filtration material.

The negative pressure induced by the fan 33 is set for example down toapproximately −50 Pa.

FIG. 6 shows how the flow is controlled.

The flow has a reducing cross sectional area from the inlet 30 to theoutlet 32, hence an increasing velocity along its axis. The air flow 50through the filters 44 focuses the particle trajectory, and there is anacceleration towards the outlet 32 within the measurement zone 52 wherethe measurement takes place.

FIG. 7 shows the flow velocity profile in the measurement zone 52, as aplot of velocity versus distance along the flow path.

FIG. 8 shows the effect of particle density (which correlates to theeffective aerodynamic diameter) on the velocity reached within theacceleration zone 52. It shows the relative velocity versus the distancefrom the inlet nozzle for a set of different particle densities rangingfrom 1.0 g/cm³ to 2.0 g/cm³ for particles with identical diameter of 10μm, and uses particles of density 1.0 g/cm³ as a baseline reference.

From this graph, the velocity difference with respect to the baselinecan be determined (such as the vertical arrow 80 shown) at differentdistances from the inlet nozzle. The reference line (v=0) indicates thereference with particles having density 1 g/cm³, and thus is set to 0 atall distances.

The other plots show the velocity difference for particles with density1.2, 1.5, 1.8 and 2.0 g/cm³ compared to the baseline reference. Largerparticles accelerate more slowly than smaller particles, thus particleswith higher density (i.e. larger aerodynamic diameter) have a morenegative value as shown by arrow 80 for the most dense particle.

After the accelerating flow field, in the measurement zone, the velocityprofile of the flow changes, so that the velocity differences thendiminish and all particles settle in the flow field with the sameterminal velocity. This is also shown in FIG. 8.

As explained above, the total time a particle traverses the measurementzone (i.e. the width of the signal Δt in FIG. 4) is used to characterizethe aerodynamic diameter of the particle. To estimate the potentialaccuracy and stability of this algorithm, a sensitivity analysis isperformed for the two relationships (i) terminal velocity to particleeffective aerodynamic diameter, and (ii) transit time (in themeasurement zone) to particle effective aerodynamic diameter.

The sensitivity (x) can be defined as [(dy/y)/(dx/x)] (x) for arelationship y=f(x).

This sensitivity defines the ratio of percentage of change in y andpercentage of change in x at different values of x. A high sensitivitymeans that a change in x will cause a prominent change in y. Thus bymeasuring y, a more accurate determination of the value of x can beobtained with less interference from noise.

The sensitivity analysis is performed for particles with aerodynamicdiameter ranging from 1 μm to 100 μm in steps of 1 μm and a uniformdensity 1.5×10³ kg/m³. The measurement zone length is 4 cm.

FIG. 9 shows the terminal velocity v (m/s) versus the particle diameter(m).

FIG. 10 shows the sensitivity S (unitless) of the terminal velocityversus the particle diameter (m).

FIG. 11 shows the pulse width W (ms) versus the particle diameter (m).

FIG. 12 shows the sensitivity S (unitless) of the pulse width versus theparticle diameter (m).

In FIG. 10, the sensitivity of the relationship particle between theterminal velocity and particle effective aerodynamic diameter rangesfrom 0 to 0.5 (absolute value) with higher sensitivity for largerparticles. The same conclusion can be drawn from FIG. 12 where thesensitivity of the relationship between particle transit time andparticle effective aerodynamic diameter ranges from 0 to 0.25, also withlarger particles having higher sensitivity.

This sensitivity analysis shows that the use of particle transit time tocharacterize particle aerodynamic is feasible and more sensitive (givinga more accurate and stable estimation) for larger particles.

The nature of the movement of the particles in the flow can be analyzedbased on the drag force experienced by the particles.

The drag force is given by:

$\begin{matrix}{F_{drag} = \frac{3{{\pi\eta}V}_{set}d_{p}}{C_{c}\left( d_{p} \right)}} & (1)\end{matrix}$

For a particle of interest:

$\begin{matrix}{{\rho_{p}\frac{\pi}{6}d_{op}^{3}g} = \frac{3{{\pi\eta}V}_{Set}d_{op}}{C_{c}\left( d_{op} \right)}} & (2)\end{matrix}$

For a particle with standard density:

$\begin{matrix}{{\rho_{0}\frac{\pi}{6}d_{ae}^{3}g} = \frac{3{{\pi\eta}V}_{Set}d_{ae}}{C_{c}\left( d_{ae} \right)}} & (3)\end{matrix}$

These two relationships can be combined to yield:

$\begin{matrix}{d_{ae} = {d_{op}\sqrt{\frac{1}{}\frac{\rho_{p}}{\rho_{0}}\frac{C_{c}\left( d_{op} \right)}{C_{c}\left( d_{ae} \right)}}}} & (4)\end{matrix}$

ρ_(p) is the particle densityρ₀ is the standard densityη is the dynamic viscosity of air=1.893×10⁻⁵ Pa·sχ is the dynamic shape factor (1 for a sphere, <1 for streamlinedshape, >1 for most aerosol particles)V_(set) is the settling velocity (the relative velocity of the particlewith respect to the carrier flow)C_(c)(d_(p)) is slip correction factor, which has a known relationshipwith respect to d_(p) and can be approximated to 1 for particles largerthan 1 micrometerd_(op) is the effective optical diameterd_(ae) is the effective aerodynamic diameterd_(p) is the generic particle diameter value used in the generic dragequation.

There are different options for processing the recorded data.

In a most basic version, the height and width of the scattered signal ofFIG. 4 are measured, where the height of the signal is related toparticle effective optical diameter, and the width of the signal (thetransit time of the particle in the laser beam) is related to theeffective particle aerodynamic diameter by the relationship shown inFIG. 10.

After obtaining the effective optical and aerodynamic diameters, thevalue under the root sign of Equation (4) can be derived for a specificparticle.

In addition, as Cc(d_(ae)) and Cc(d_(op)) have known relationship withdependent variable d (i.e. d_(ae) and d_(op)) and are close to one formicron-sized particles, the ratio of density (ρ_(p)) and shape factor(χ) can be calculated as a single number.

This most basic approach provides one additional parameter whichcombines density and shape factor. This may be used to provide particledifferentiation in that different particles will have different valuesfor this parameter.

In this basic version, the measured variables are the effective opticaldiameter (derived from peak intensity) and the effective aerodynamicdiameter (derived from the transit time). These are outputs from thesystem.

Known variables are ρ₀=1 g/cm³, Cc(d_(op))≈1, Cc(d_(ae))≈1.

The calculated additional variable is the ratio of particle density(ρ_(p)) to shape factor (χ). This is also provided as an output. Thesystem can also output particle number concentration.

It would be more useful to be able to separate the particle density andshape information. Thus, in a more advance implementation, a particleshape factor may be obtained by analyzing a variation of the scatteredlight signal collected by the photon detector.

FIGS. 13 and 14 are used to explain a first approach.

FIG. 13 shows a collected signal for a non-spherical particle. Theorientation of the particle influences the scattered light intensity inthe direction of the photon detector, and this manifests itself as avariation in the intensity level during the period of the signalcorresponding to the central area of the measurement zone.

FIG. 14 shows a collected signal for a spherical particle. Theorientation of the particle does not influence the scattered lightintensity in the direction of the photon detector, so the peak intensityhas a period of constant amplitude. In each case, the peak intensity 120recorded for the purposes of determining the effective optical diameteris the same.

A shape factor x can then be evaluated and hence the particle densitycan be separated from the shape factor. The deviation of the signal ofFIG. 13 from a constant peak value may be obtained by any suitablestatistical analysis, such as the peak deviation from the baseline level(shown dotted) or an area of deviation.

FIG. 15 is used to explain a second approach.

The optical system is enhanced by providing a first polarizer 150 in thepath of the laser beam, before the measurement zone, and a secondpolarizer 152 is provided in the path to the photon detector 38.

The second polarizer has two portions 152 a, 152 b with orthogonalpolarization. The first part 154 of the path of the particle within themeasurement zone is detected based on light which has passed through thefirst portion 152 a, and the second part 156 of the path of the particleis detected based on light which has passed through the second portion152 b. In this example, the first portion has a polarization alignedwith the first polarizer, and the second portion has a polarizationorthogonal to the first polarizer.

FIG. 16 shows an example of the collected light intensity for anon-spherical particle.

During a first part of the time period a light intensity is measured forlight that has passed though aligned polarizers, with a maximumintensity shown as I_(max)∥. During a second part of the time period alight intensity is measured for light that has passed though crossedpolarizers, with a maximum intensity shown as I_(max) _(⊥) . In thiscase, only light that has undergone a polarization rotation between thepolarizers can be collected.

For spherical particles, the scattered light will maintain its originalpolarization along each of the two orthogonal directions. Fornon-spherical particles, a portion of the scattered light willexperience a change in polarization, hence yields a cross contributionof polarized components between the two orthogonal directions.

The two polarized components of the scattering field depend on theS-matrix

$\begin{pmatrix}E_{s} \\E_{\bot s}\end{pmatrix} = {{\frac{e^{{ik}{({r - z})}}}{- {ikr}}\begin{bmatrix}S_{2} & S_{3} \\S_{4} & S_{1}\end{bmatrix}} \cdot \begin{pmatrix}E_{i} \\E_{\bot i}\end{pmatrix}}$

For spherical particles, S₃=S₄=0, so that the two polarizationcomponents in the incident and scattered light will not interact. Fornon-spherical particles, S3=S4≠0, the two polarization components willinteract. i.e. the parallel component in the incident light willcontribute to both parallel and perpendicular components in scatteredlight.

A shape measure may be defined as:

${P \equiv \frac{{Imax}_{} - {Imax}_{\bot}}{{Imax}_{} + {Imax}_{\bot}}} = {f()}$

For spherical particles, I_(max⊥)=0⇒P=1. For non-spherical particles,Imax_(⊥)>0 ⇒P<1.

Thus, the parameter P provides a measure of the shape of a particle fromwhich an estimate is obtained for shape factor χ and hence the particledensity can be separated from the shape factor.

In this approach, the measured variables are the effective opticaldiameter (based on the peak intensity), the effective aerodynamicdiameter (based on the transit time) and the shape factor (χ) (based onthe shape of the intensity plot). These are outputs by the system.

The known variables are again ρ₀=1 g/cm³, Cc(d_(op))≈1, Cc(d_(ae))≈1.

The calculated variable is the particle density (ρ_(p)) which is alsooutput from the system, again also with a particle count.

The retrieval of the effective aerodynamic diameter as well as the shapefactor using either of these approaches requires a wide incident laserbeam (of the order of millimeters or centimeters) compared to a typicalnarrow laser beam (of the order of micrometers).

The left part of FIG. 17 shows parameters associated with the opticalarrangement of FIG. 3, in particular the laser beam width d and the pathlength L in the measurement zone. The angle between the incident beamand the flow path is shown as θ_(inc). The scattering angle between theincident beam and the photon detector is shown as θ_(sca).

The right part of FIG. 17 shows the effect of decreasing the angle ofincidence.

For a given path length L and laser power, and beam width d can bedecreased, so that the irradiance can be increased giving a highersignal to noise ratio.

The scattering angle θ_(sca) can also be decreased to increase thescattering intensity reaching the photon detector, as there is moreforward scattering for particles with size larger than 0.5 μm.

By way of example, preferred ranges for θ_(inc) are 15 to 25° such as20°. Preferred ranges for θ_(sca) are 20 to 40° such as 30°. The lengthL has the order of centimeters, such as 1 cm to 6 cm, such as 2 cm. Asuitable range for d is L/3 to L/4.

The system will be calibrated with respect to:

The signal height to effective optical diameter relationship;

The signal width to effective aerodynamic diameter relationship;

The signal variation to shape factor relationship (when used); and

The parameter P to shape factor relationship (when used).

FIG. 18 shows a method for obtaining particle characteristics,comprising:

in step 180, generating an accelerating flow containing particlesbetween an inlet and an outlet;

in step 182, controlling a laser light source to provide light to ameasurement zone of the flow path;

in step 184, detecting light received from the measurement zone whichhas been scattered by the particles;

in step 186, determining a time duration for which a particle remains inthe measurement zone and thereby determining an effective aerodynamicparticle diameter;

in step 188, determining a peak detected received light intensity andthereby determining an effective optical particle diameter; and

in step 190, determining a further particle parameter relating to theshape and/or density of the particle.

As discussed above, embodiments make use of a controller 41. Thecontroller can be implemented in numerous ways, with software and/orhardware, to perform the various functions required. A processor is oneexample of a controller which employs one or more microprocessors thatmay be programmed using software (e.g., microcode) to perform therequired functions. A controller may however be implemented with orwithout employing a processor, and also may be implemented as acombination of dedicated hardware to perform some functions and aprocessor (e.g., one or more programmed microprocessors and associatedcircuitry) to perform other functions.

Examples of controller components that may be employed in variousembodiments of the present disclosure include, but are not limited to,conventional microprocessors, application specific integrated circuits(ASICs), and field-programmable gate arrays (FPGAs).

In various implementations, a processor or controller may be associatedwith one or more storage media such as volatile and non-volatilecomputer memory such as RAM, PROM, EPROM, and EEPROM. The storage mediamay be encoded with one or more programs that, when executed on one ormore processors and/or controllers, perform the required functions.Various storage media may be fixed within a processor or controller ormay be transportable, such that the one or more programs stored thereoncan be loaded into a processor or controller.

The invention makes use of a wide light source illumination beam andenables effective aerodynamic diameter, effective optical diameter andshape factor all to be achieved in one shot. With this design, thesensor can be made with a small footprint and reasonable sensitivity(compared to a professional mass spectrometry system).

The examples above show a measurement zone with continuous illumination.An alternative is to splitting a beam into two to define an envelopewith two portions. The length of the measurement zone can then beincreased in return for a higher sensitivity for aerodynamic diametermeasurement, while retaining the shape determination function. Forexample a 4 cm beam may be split into a 2 cm laser+2 cm void+2 cm laserenvelope with a measurement zone length of 6 cm.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measures cannot be used to advantage. Any reference signs inthe claims should not be construed as limiting the scope.

1. An apparatus for determining particle characteristics, comprising: aninlet and an outlet; a flow system for generating a flow path betweenthe inlet and the outlet along which an accelerating flow is to beprovided containing particles to be analyzed; a light source forproviding light to the flow path, wherein the envelope of the lightdefines a measurement zone, the measurement zone having a length of morethan 1 cm; a light detector for detecting light received from themeasurement zone which has been scattered by the particles; and acontroller for analyzing the detected received light, wherein thecontroller is adapted to: determine a time duration for which a particleremains in the measurement zone, and thereby determine an effectiveaerodynamic particle diameter; determine a peak detected received lightintensity and thereby determine an effective optical particle diameter;determine a further particle parameter relating to the shape and/ordensity of the particle.
 2. An apparatus as claimed in claim 1, whereinthe controller is adapted to: determine the further particle parameteras a ratio between a particle shape parameter and a particle density. 3.An apparatus as claimed in claim 1, wherein the controller is adaptedto: analyze a level of variation of the detected received intensity overtime and thereby derive a particle shape parameter as a first furtherparticle parameter; and determine a particle density as a second furtherparticle parameter.
 4. An apparatus as claimed in claim 1, furthercomprising: a first polarizer between the light source and themeasurement zone; and a second polarizer between the measurement zoneand the light detector, wherein the second polarizer has a first portionwith a matching polarization to the first polarizer and a second portionwith an orthogonal polarization to the first polarizer.
 5. An apparatusas claimed in claim 4, wherein the controller is adapted to: analyze thepeak detected received light intensity through the first and secondportions of the second polarizer to derive a particle shape parameter asa first further particle parameter; and determine a particle density asa second further particle parameter.
 6. An apparatus as claimed in claim1, wherein the flow system comprises a fan connected to the outlet, andwherein the apparatus further comprises: an outer enclosure; a filterarrangement coupled to a further pair of inlets; and a flow deflectorarrangement for controlling the flow from the filter arrangement and forcontrolling the flow path.
 7. An apparatus as claimed in claim 6,wherein the filter arrangement comprises first and second filters onopposite sides of the flow path, and the flow deflector arrangementcomprises corresponding first and second flow deflectors.
 8. Anapparatus as claimed in claim 1, adapted to provide a uniformacceleration of the flow along the flow path within the measurementzone.
 9. A method for obtaining particle characteristics, comprising:generating an accelerating flow containing particles along a flow pathbetween an inlet and an outlet; controlling a light source to providelight to the flow path, wherein the envelope of the light defines ameasurement zone, the measurement zone having a length of between 1 cmand 6 cm; detecting light received from the measurement zone which hasbeen scattered by the particles; determining a time duration for which aparticle remains in the measurement zone and thereby determining aneffective aerodynamic particle diameter; determining a peak detectedreceived light intensity and thereby determining an effective opticalparticle diameter; and determining a further particle parameter relatingto the shape and/or density of the particle.
 10. A method as claimed inclaim 9, comprising: determining the further particle parameter as aratio between a particle shape parameter and a particle density.
 11. Amethod as claimed in claim 9, comprising: analyzing a level of variationof the detected received intensity over time and thereby deriving aparticle shape parameter as a first further particle parameter; anddetermining a particle density as a second further particle parameter.12. A method as claimed in claim 9, comprising: providing polarizationof the light between before the measurement zone using a firstpolarizer; providing polarization of the light after the measurementzone using a second polarizer having a first portion with a matchingpolarization to the first polarizer and a second portion with anorthogonal polarization to the first polarizer; analyzing the peakdetected received light intensity through the first and second portionsof the second polarizer to derive a particle shape parameter as a firstfurther particle parameter; and determining a particle density as asecond further particle parameter.
 13. A method as claimed in claim 9,comprising providing a filtered air flow towards the measurement zonethereby to control the flow path.
 14. A method as claimed in claim 9,comprising providing a uniform acceleration of the flow along the flowpath within the measurement zone.
 15. A computer program comprisingcomputer program code means which is adapted, when said computer programis run on a computer, to perform the method of claim 9.